Industrial blast nozzles, which will hereinafter and in the appended claims be referred to simply as “blast nozzles”, are used to direct abrasive-bearing fluids against workpieces for purposes ranging from preparing or conditioning a workpiece surface to machining or cutting apart the workpiece. The fluid may be a gas, such as compressed air, or a liquid, such as high pressure water. The abrasives are in particulate form and their physical properties are selected based upon the type of job to be performed and the characteristics of the workpiece. Blast nozzles are sometimes classified by their design and operational mode as venturi nozzles, straight bore nozzles, angle nozzles, and wet blast nozzles.
Blast nozzles are subjected to intense internal abrasion from the abrasive flow they carry and direct, making it necessary to construct them, at least in part, from wear-resistant materials, e.g., cermet and ceramic materials. One such ceramic material is SiAlON, a lightweight ceramic that offers a service life and durability similar to the much heavier cermet tungsten carbide. This weight-reduction advantage reduces operator fatigue and has made SiAlON a popular choice as a blast nozzle material in the last decade. Examples of SiAlONs are described in U.S. Pat. Nos. 4,127,416, 4,547,470, 4,563,433, 4,711,644, 4,826,791, 4,880,755, 5,059,768, 5,066,423, 5,108,659, 5,118,646, 5,200,374, 5,302,329, 5,413,972, 6,124,225, 6,471,734 B1, 6,693,054 B1, 7,223,709 B2, 7,309,673 B2, and 7,094,717 B2.
However, a drawback to the use of SiAlON is that it generally lacks the electrical conductivity of tungsten carbide. In some cases, SiAlON's poor electrical conductivity results in the buildup on a blast nozzle of undesirable levels of static electrical charge resulting from the sliding contact of the abrasive particles flowing through it. This can lead to undesirable and uncontrolled electrical spark discharges. The static discharge problem has limited the use of the SiAlONs in blast nozzle applications to those applications in which some unexpected electrical spark discharge can be safely tolerated.
In the 1980's and early 1990's, work was done on modifying SiAlONs to render them sufficiently electrically conductive to be machined by electric discharge machining (EDM) or to make them suitable for use as electrical heating elements or glow plugs. Examples of such work are disclosed in U.S. Pat. Nos. 5,059,768, 5,066,423, and 5,108,659. The first and last patents of this group teach that improved electrical conductivity may result from the addition to the SiAlON-containing ceramic of at least one of titanium nitride, titanium carbide, and titanium carbonitride. The remaining patent in the group greatly expands the list of possible additives to include one or more conductive compounds of carbides, nitrides, oxides and their composite compounds of transition metals in Groups IVa, Va, and VIa of the Periodic Table in conjunction with an addition of the less conductive material silicon carbide to lessen the batch-to-batch, sample-to-sample, and even intra-sample variability of electrical resistivity. However, although such efforts improved the usability of SiAlONs in electrical heating applications, abrasion resistance is not a concern in such applications and so there was little interest in measuring the wear resistance properties of such modified SiAlONs. Indeed, none of the patents in the above-mentioned group provide any teachings at all on the abrasive resistance characteristics of these modified SiAlONs. Thus, until now, two decades later, these modified SiAlONs have not been used in applications requiring a high level of wear resistance.